Poly(vinylidene fluoride) (PVDF) is obtainable via polymerization of vinylidene difluoride monomers (difluoro 1,1-ethylene, VF2 or VDF). Poly(vinylidene fluoride) can be advantageously used in several different applications for the manufacturing of piping products, sheets, tubing, films, plates, membranes and as an insulator for premium wire.
The main advantage of the VDF suspension polymerization is the absence of any fluorinated surfactant.
During suspension polymerization of VDF in the absence of a suspending agent, reactor fouling (that is build-up of polymer deposits on the inner surfaces of the reactor and of the stirring equipment) is observed, which prevents the scaling of the process to industrial production.
The amount of fouling may be assessed via visual inspection and expressed on an arbitrary scale 0 to 4, wherein a degree of 2 or higher indicates too much fouling for the process to be industrialized. Although the VDF polymers produced via polymerization in suspension are very white, they tend to turn brown when exposed to concentrated acids and to acid solutions for longer than 8-10 hours. This visual drawback of PVDF is perceived as detrimental and limits the use of PVDF in many applications.
EP2397519 discloses a vinylidene fluoride resin film including a vinylidene fluoride resin, a methacrylic acid ester resin, a titanium oxide surface treated with alumina and silica, and a fatty acid ester of polyethylene glycol and/or a derivative thereof. EP2397519 has the main object to provide a vinylidene fluoride resin film that, with respect to the earlier multilayer structure and films, has improved adhesiveness to a base material, achieves improved dispersivity also when a pigment is contained in large amounts, and has excellent thermal stability during a forming process.
WO2012084580 pertains to a grafted fluorinated polymer comprising at least one grafted side chain comprising one or more glycosidic recurring units, formed via a process comprising polymerizing VDF, optionally in the presence of one or more other fluorinated monomers, or one or more (meth)acrylic monomers, in the presence of at least one polysaccharide derivative, that is notably aimed at providing improved hydrophilic membranes, e.g. suitable for water purification.
One aim of the present invention is to provide a PVDF polymer having improved colour stability when exposed to acids and a process for its production.
Another object of the present invention is also to provide a method for manufacturing a PVDF polymer that is feasible and easy to scale for industrial production. It is desirable that the polymer suspension does not generate any fouling during the polymerization and that all the advantageous properties of the polymer are retained.